Bimetallic coupling joint for tubes of dissimilar materials

ABSTRACT

A bimetallic metallurgical joint between tubes having widely differing coefficients of thermal expansion particularly wherein one tube comprises a low-coefficient-expansion refractory metal and the other tube is of a conventional alloy. The joint is designed to maintain its integrity while accommodating repeated severe thermal cycles.

United States Patent m1 3, 32,143

[72] Inventor Gerald G. Lessmann [56] Relerences Cited 1 A l N 53 23?UNITED STATES PATENTS E f 19 1969 2,312,909 3/1943 Jeffery 285/187 [45]Patented n.4, 3,311,392 3/1967 Bu schow 277/236 X Assignee WestinghouseElecmc Corporation 3,411,812 1 l/l968 Prince et al 27/26 X Pittsburgh,Pl. Primary Examiner-Dave W. Arola Attorneys-F. Shapoe and L. P. Johns[54] BIMETALLIC COUPLING JOINT FOR TUBES OF DISSIMILAR MATERIALSABSTRACT: A bimetallic metallurgical joint between tubes 13 Claims, 4Drawing Figs. having widely differing coefficients of thermal expansionpar- [52] U 5 Cl ticularly wherein one tube comprises alow-coefficient-expan- 277/26, Sion mfractory metal and the other tubeis of a conventional [51] Int Cl 55/00 alloy The joint is designed tomaintain its integrity while ac- [501neidoiQQBIIIIIIIIIIIIIIII'"""""""'IIIIIIIIIIII 285mmcommodafinsrepmdsevflelhmlcym 50 f 40 72 52 l5 a may,

PATENTEUJM 4m: 131632.143

SHEET 1 OF 2 WITNESSES INVENTOR W Gerald G. Lessmonn BIMETALLIC COUPLINGJOINT FOR TUBES OF DISSIMILAR MATERIALS BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to a bimetallic joint forconnecting tubular members such as piping, tubing, conduits, or thelike, having very dissimilar coefficients of thermal expansion suitablefor use at temperatures of up to about 2,500 F. for carrying steam, hotgases, NaK, and so on. More particularly, the invention pertains to atight, reliable joint between pipes of a reactive or refractory metalalloy and more conventional piping metals and alloys, having widelydiffering coefficients of thermal expansion.

2. Description of the Prior Art There is a recurring need for a reliableleaktight connection or joint between tubes of difierent materials suchas metals having substantially different coefficients of thermalexpansion. Joints between refractory metals, such as base alloys ofcolumbium or tantalum and more conventional nonrefractory metals, suchas stainless steel, have presented a recurring problem in the art.Previous bimetallic joints have not been satisfactory for the hightemperatures which are encountered in typical applications involvingrefractory metal alloys. That is particularly true where repeated severetemperature cycles occur.

In general, conventional simple mechanical joints have been unacceptableand are relatively unsatisfactory because of the severity of theenvironment coupled with the requirement for leaktightness andreliability. Brazing is of limited value for joining metals of suchwidely dissimilar characteristics because of the specializedenvironmental tolerance of the brazing alloys and specialized behaviorachieved in brazed joints. This is in addition to the fact that brazedjoints cannot be opened or broken readily and reclosed. That is, thebrazing alloy represents a metallurgical discontinuity between twodissimilar materials which are selected to perform specializedfunctions, often at the limit of their capabilities. These functionscoupled with the severe environmental resistance required in suchapplications cannot be satisfied in a general way by brazing. Finally,other methods of joining requiring the application of heat to themelting point of metals such as welding, leads to extensive formation ofbrittle intermetallic compound layers in refractory metal pipesresulting in mechanically unacceptable joints which often fracture oncooling from the welding temperature or fracture during normal handlingor use.

Associated with the foregoing considerations is the problem of repeatedthermal cycling. The joints of prior construction often have been unableto maintain their integrity when subjected to repeated severe thermalcycles. That is particularly true where the joint is provided betweenincompatible materials having widely differing coefficients of thermalexpansion.

SUMMARY OF THE INVENTION It has been found in accordance with thisinvention that the foregoing problems may be overcome by providing abimetallic coupling joint for connecting a pipe of a refractory metalwith a pipe of a more common metal, which metals have a widely differentcoefficient of thermal expansion, and said coupling joint comprising apair of gasket-forming sheets of metal having substantially difierentcoefficients of thermal expansion and being prebonded together at theirinterfaces to provide an intermetallic interface having a minimumthickness. The pair of gaskets are disposed between spaced end surfacesof flanges on the conduits which are to be joined together, the endsurfaces of the flanges being inclined radially outwardly. The conduitsare fastened together on opposite sides of the prebonded sheets byconnection means attached to the flanges which means have a coefficientof thermal expansion substantially less than that of the conduit metalhaving the higher coefiicient of expansion.

Accordingly, it is a general object of this invention to provide abimetallic joint for dissimilar metals which includes a combinedmetallurgical-mechanical sealed joint.

It is another object of this invention to provide a bimetallic joint forpipes of dissimilar metals which maintain its integrity whileaccommodating repeated severe thermal cycling.

It is another object of this invention to provide wide designflexibility in a coupling joint to accommodate widely diverse systemsinvolving cyclic thermal and load conditions while simultaneouslymaintaining reliable joint integrity.

It is another object of this invention to provide a bimetallic gasketjoint for pipes of metal having widely different coefficients of thermalexpansion which maintains a constant pressure on the joint over a widerange of temperature cycles.

Finally, it is an object of this invention to satisfy the foregoingobjects and desiderata in an effective manner.

Briefly, the invention provides a coupling between a pair of conduits ofsubstantially different coefficients of thermal expansion, thedifference being of the order of at least 4 microinches per inch per F.,comprising first and second conduits having inner and outer tubularwalls, the first conduit having a higher coefficient of thermalexpansion than the second conduit, the first conduit having acone-shaped or dished end surface inclined at an angle to a verticalradial plane through said conduit, a second conduit having an endsurface inclined at a substantially matching angle to the radial plane,a flexible metallic gasket interposed between said end surfaces andcomprising superposed layers of metals having different coefficients ofthermal expansion with the metal layer having the higher coefficient ofexpansion being disposed adjacent to the first conduit, the layers ofmetal having metallurgically bonded interfaces joined in a leakproofmanner, and mechanical connection means for holding the inclined endsurfaces in surface-to-surface contact with adjacent layers of thegasket, whereby the coupling maintains a fluidtight joint and a stablemechanical relationship between the first and second conduits regardlessof the differential radial and axial contraction and expansion of theconduits during repeated large changes in temperatures and pressure of afluid flowing through the conduits.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thenature and objects of this invention, reference is made to the drawings,in which similar numerals refer to similar parts throughout the severalviews to the drawings, and in which:

FIG. I is an illustration of the preferred embodiment of the invention,showing the upper portion in vertical section and a lower portion inelevation;

FIG. 2 is a longitudinal sectional view through a portion of a coupledpipe showing another embodiment of the invention; and

FIGS. 3 and 4 are fragmentary longitudinal sectional views showingadditional embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 a joint or couplinggenerally indicated at 10 is disposed between a pair of tubes orconduits 12 and 14 composed of materials having substantially differentcoefficients of thermal expansion. The coupling 10 is provided foraccommodating conduits having widely different coefficients of thermalexpansion for conducting fluids over a wide temperature range from belowroom temperature up to the maximum useful temperature of the weaker (attemperature) alloy. For example, the conduit 12 may be composed of anymaterial such as a metal or alloy having a high or relatively highcoefficient of thermal expansion such as the austenitic type of steelgenerally designated as A181 321, the coefficient of expansion of whichis 10.7 microincheslinchl F. The conduit 14 may be composed of anymaterial such as a metal or alloy having a low or relatively lowcoefficient of thermal expansion such as Cb-lZr (columbium-l percentzirconium), the coefiicient of expansion of which is 4.4microinches/inch/ F., the difference in the thermal expansions beingabout 6.3 microinches per inch per F. However, the coupling will besuitable for connecting satisfactorily conduits of metals whosecoefficients of thermal expansion differ by less than 4 microinches perinch per F.

The coupling preferably includes a pair of flangelike members, such as afirst flange 16 forming a part of conduit 12, a second flange 18 forminga part of conduit 14, gasket means 20, and joining or clamping means forholding the flanges together in surface-to-surface contact with thegasket 20, such as nut and bolt assemblies 22.

The flanges l6 and 18 are each composed of a material which is the sameas or similar to the corresponding conduits l2 and 14. For example, theconduit 12 having a high coefficient of thermal expansion compared tothat of the conduit 14, is provided with the flange 16 of AlSl 321 alloyor similar austenitic steel having the same or substantially the samecoefflcient of thermal expansion. Similarly, the flange 18 comprisesCb-lZr alloy or similar alloy having a coefficient of thermal expansionwhich is the same or substantially similar to that of the conduit 14.The flanges l6 and 18 are attached at annular portions 28 and 29 to theconduits 12 and 14 by welds 24 and 26 respectively. Where the materialsof the conduits 12 and 14 are composed of materials that are suitablefor forming at elevated temperatures such as by forging or upsettingprocedures, the flanges l6 and 18 my be formed as an integral part ofthe conduits l2 and 14, whereby the welds 24 and 26 are unnecessary.

The flange l6 constitutes a thickened or enlarged extension of theconduit 12 with common inner surfaces 30 and outer surfaces 32. Theflange 16 includes a fillet 34 joining the outer surface 32 and anexternal side surface 36 of the flange. In addition, the flange 16includes an end surface 38, an outer peripheral surface 40, and arecessed inner surface 42 beginning beyond the annular portion 28. Theflange 16 also includes a conically inclined or dished end surface 44which end surface extends from the end surface 38 to the recessed innersurface 42. The end surface 44 is inclined at an angle to a verticalplane through the conduit so that it extends from the inner surface 30toward the flange 18; that is, the end surface 44 is inclined outwardlyto the right as viewed in F IG. 1.

The flange 18 has an extension 29 welded at 26 and having a common innersurface 56 and outer surface 46 with pipe 14. Flange 18 is injuxtaposition to the flange 16. The flange 18 includes an external sidesurface 48, a peripheral surface 50, and an end surface 52, the last ofwhich is spaced from and faces the annular side surface 38 of the flange16. In addition, the flange 18 includes an inner surface 54 which isrecessed radially outwardly from the inner surface 56 of the conduit 14.For reinforcement the flange 18 includes an enlarged or thickened flangeportion 58 which is bounded by a radial side surface 60 and peripheralsurface 62. In the alternative the enlarged portion 58 may be omittedand a fillet 64 (shown in dotted lines) may be substituted in a mannersimilar to the fillet 34 of the flange 16. Moreover, if necessary, forreinforcement of the flange 16 an enlarged portion similar to theportions 58 may be substituted for the fillet 34.

The flange 18 also includes an outwardly coned inclined surface 66 whichextends inwardly from the side surface 52 to the surface 54, a junctionline 68 formed where the surface 52 and 66 meet is axially spaced from asimilar line 70 formed where the surface 38 and 44 meet on the flange16.

As shown in H6. 1 means are provided for holding the joint together,comprising a plurality of nut and bolt assemblies 22 disposed at spacedintervals around the flanges 16 and 18 and which extend through alignedapertures 72 and 74 in the flanges 16 and 18, respectively, and includesa washer 76 adjacent the external side surface 36 of the flange 16.Where the assembly 22 and the flange 16 are composed of suchcompositions that they will interact or diffuse together particularly athigher temperatures of operation, the washer 76 may be an inertcomposition that prevents such interaction. A similar washer may beapplied between the head of bolt 22 and the external side surface 48 onflange 18.

The gasket means 20 includes at least two cone-shaped layers 78 and 80of annular configuration disposed between the surfaces 44 and 66. Thelayers 78 and 80 have thickness range of from about 0.015 inch to about0.25 inch, the minimum total thickness being about 0.030 inch. However,the minimum thickness is primarily dependent upon ease of handling andfabrication. Fabrication costs rise sharply with thinner gaskets. Thoughthe surfaces 44 and 66 are preferably parallel, they may be slightlytapered with respect to each other, or otherwise shaped for preventinginternal fluid pressures within the conduits 12 and 14 from blowing thegasket layers 78 and 80 radially outwardly or to achieve locally higherclamping pressures on selected areas of the gasket layers.

More particularly, it is important that the layers 78 and 80 be composedof materials that are compatible in welding with the materials of whichthe corresponding flanges or tubular extension 88 and 90 are composedfor the reasons indicated below.

The layer 78 is preferably composed of a material similar to thematerial of the flange 16; or in the alternative, the layer 78 iscomposed of a material having a coefficient of linear thermal expansionsubstantially corresponding to that of the flange 16.

The layer 80 is composed of a material similar to the flange 18; or, inthe alternative, is composed of a material having a relatively highcoefficient of linear thermal expansion substantially corresponding tothat of the flange 18.

The layers 78 and 80 may be composed of various combinations ofmaterials, and illustrative examples which have been evaluated arelisted in the following table. For example, when the conduit 12 and theflange 16 are composed of metals having the same or substantiallysimilar coefficients of thermal expansion (a relatively highcoefficient), the layer 78 is composed of a metal of the same orsubstantially the same coeffcient of thermal expansion. The samerequirements pertain to the layer 80, the flange l8, and the conduit 14.

TABLE Composition of Some Gasket Layer Combinations of Refractory andConventional High Temperature Materials Cb/SZl (AISU Cit/347 (AlSl)Cbllnconel 600 Cb-lZr/347 (AISI) Cb-lZr/Inconel 600 Ta/32l (AlSl) TB/347(AlSl) Ta/Inconel 600 Tall-lastelloy N FS-/32l (AlSl) FS-85/347 (AlSl)FS-SSJlnconel 600 FS-SS/Hastelloy N T-222/32l (AlSl) T-222/347 (AlSl)T-222/lnconel 600 Nominal Compositions AISI 32l-(Fe-l8Cr-l 0Ni-0.08CMax-Ti stablilized) AlSl 347-1 Fe-l 8Cr-l lNi-ODBC MaxCb stabilized)Hastelloy N-(Ni-l 7Mo- 7Cr-5FE-0.06-O. l 5-0.5 AL)FS-BS-(Cb-27la-1OW4Zr) To produce a fluidtight joint between the layers78 and 80 the interfaces of the layers are metallurgically bondedtogether in a leakproof manner. The bonding is accomplished by providinga metallurgical joint in any suitable manner such as diffusion bonding,gas-pressure bonding, explosive cladding, or a combination of theseprocesses. The several combinations of layers listed in the table wereexplosively bonded. The resulting bonded zone or ares 82 is an areawhere the different metals have interdiffused, intermixed, or formedintermetallic compounds, having a thickness of up to 0.5 mil, andpreferably less than 0.2 mil. Bonded-zone thicknesses of greater than0.5 mil may deteriorate and crack due to sever thermal cycling in use;the smaller the thickness of the zone the better. In the combinations ofthe table, a joint thickness of 0.2 mil or less was found to beexcellent.

An explosive bond is preferred over any other type such as generallyachieved with brazing and welding since it avoids excessiveinterdiffusion between the gasket metal layers and hence excessivedevelopment of an intennetallic zone at the interface. lnterdifiusionduring subsequent use at elevated temperatures such as occurs attemperatures of up to l ,500" F. and higher is also undesirable.Interdiffusion between the materials constituting the bonded layers 78and 80 is undesira ble in order to avoid the formation of enlarged zonesof intermetallic compounds which result in brittleness and mayeventually cause failure of the bond 82 when subjected to strain causedby thermal cycling.

By proper selection of materials for the assembly 22 the maintenance ofa nearly constant pressure on the gasket is possible during thermalcycling between low and elevated temperatures of operation. The assembly22 in the preferred embodiment is composed of a metal having a lowcoefficient of linear thermal expansion comparable to that of the flange18 and/or the conduit 14. In the case of a joint between a refractorymetal and convention alloy suitable for piping the low expansion alloyis also generally the stronger at temperature and hence the best for usefor the bolt and nut assembly 22. If desired the assembly 22 may becomposed of a metal having a high expansion coefficient (comparable tothat of the flange l6), and in this case the angles of inclination ofthe end surfaces 44 and 66 are reversed to the opposite side of a radialplane extending through the pipe, that is, at an angle extending to theleft as viewed in FIG. 1.

As shown in FIG. 1 the layers 78 and 80 have flanges 84 and 86,respectively, with flange extensions 88 and 90, having a reducedthickness as compared to the flanges 84 and 86. The flanges arerelatively flexible and compensate for radial movement of the jointduring thermal expansion and contraction.

As shown the flange extension 88 is welded at 98 to the flange 82.Likewise, the flange extension 90 is welded at 100 to the flange 86. Theextremities of the flange extensions 88 and 90 are welded at 92 and 94respectively to the inner surfaces 30 and 56 of the flanges l6 and 18.The flanges extension 88 is seated within the recessed inner surface 42.Likewise the flange 86 and the extension 90 are seated within and onlypartially fill the recessed inner surface 54 of the flange 18. It ispreferred that there is a slight clearance 96 between the flange 86 andextension 90 with respect to the recessed inner surface 54 to enablemovement of the flange and extension into the clearance space at thehigher operating temperatures. Such clearance minimizes the developmentof additional stains on the bond 82 beyond those incurred due torelative local expansion differences of the two elements 78 and 80 ofgasket means 20 during thermal cycling.

While the flange extensions 88 and 90 usually will be of the samecomposition as the corresponding layers 78 and 80, the configuration ofthe layers in combination with the corresponding flanges and flangeextensions may be of a unitized L-shaped configuration as shown inFIG. 1. However, the flange extensions 88 and 90 may be composed of aslightly different metal, or they may be composed of the same materialas the corresponding layers 78 and 80 which compositions are not readilyforrnable into the L-shaped configuration, in which event they may bewelded to layers 78 and 80.

Both the angle of inclination of the bond zone 82 and the layers 78 and90 are disposed at an angle to the vertical radial plane through thepipes 12 and 14. The angle is an acute angle with respect to the conduitaxis and facing the conduit 14. A preferred angle of inclination isnecessary in order to minimize or eliminate excessive thermally inducedplastic strain on the several parts involved, namely, the layers 78 and80, the bond 82 therebetween and the assembly 22. This is necessary toavoid loosening of the coupling joint in service when thermal cyclingoccurs. By this means the gasket is mechanically restrained and thejoint continues to function properly under normal system design loads.The preferred angle is the angle theta (0) (FIG. 1) which is formed bythe intersection of an extension of the plane of the bond 82 with thecenter axis I02 and the intersection of the radial plane 104 extendingfrom the annular side surface 36 of the flange 16.

More particularly, the angle theta (for a simple case) is calculated byassuming that only two joint materials, i.e., a high and a lowcoefficient of thermal expansion metal are used for all of thecomponents. For an exemplary calculation consider any point 106 in thebond zone 82 of a gasket, which point is at a distance a from the radialplane through the surface 36 and also is at a distance r from axis 102.A relative lineal change in distance between the surfaces 44 and 66 atpoint 106 results in the axial tightening (or loosening) of the joint atsaid surfaces, which is expressed by the formula Aa=aATAa, where AT istemperature change and Au is the difference in expansion coefficients.Likewise, a radial loosening (or tightening) of the point 106 isexpressed by the formula Ar=r ATAa. From these the preferred angle thetacan be found by the formula:

tangent 0=(rATAa)/(aATAa)=r/a The angle 0 provides for exactcompensation of radial and axial compression differences such that thejoint preload in the assembly 22 is substantially maintained duringthermal cycling.

The dimension a is such that for each radial position along theinterface or bond 82 that relative motion (loosening) of the flanges l6and 18 is compensated by axial motion (tightening) to maintain arelatively constant flange loading. Hence, the bimetallic bond 82 isalways maintained under an exact preload compression. This minimizes therisk of fracturing of the interface or bond 82 and also permitsmaintaining the design preload in the nut and bolt assembly 22. Thus,only thermal stresses occur across the bond zone 82 which can be readilyaccommodated. Where the bond 82 is properly formed, that is, to have aminimal amount interrnetallics and is of a minimum thickness (less than0.2 mil) the local stresses at the bond are satisfactorily accommodated.At the inner portions of the layer 78 and 80, the flange 86 and flangeextension 90 are free to move into the clearance 96 during operation ofthe assembly at extremely elevated temperatures to facilitateaccommodation of the gross-diametrical strain across the joint.

In addition to the foregoing construction, means for retaining thegasket 20 in the desired position are provided. Such means may includeeither an annular protuberance or a series of peripherally spacedmembers 107 (as shown) extending around the clearance space between theflanges l6 and 18. Such means provides an inner axially extendingsurface 108 for the purpose of abutting and restraining the outer endsof the gasket layers 78 and in the event that said layers are forced byinternal pressures or thermal strains beyond their intended positionsbetween the surfaces 44 and 66.

The following example is illustrative of the invention:

a bimetal joint, similar to that shown in FIG. 1, is provided between apair of tubes, one tube being composed of type MS! 304 stainless steeland the other tube being composed of a columbium-base alloy containing 1percent zirconium. The tubes have an interval diameter of L5 inches anda wall thickness of 0.030 inch. The stainless steel (type 304) tube isprovided with an end flange composed of Inconel 7 l8, and the other tube(Cb-lZr) has an end flange composed of the Cb-lZr alloy.

The bolt and nut assemblies are composed of a cold-worked tantalum-basealloy (T-l l l The washers between the bolts and lnconel 718 flange arecomposed of tantalum base alloy (T-l l 1) having the surfaces adjacentto the flange being flame sprayed with A1 to avoid diffusion between thewashers and the flange.

The gasket between the flanges includes two layers of flexible metallicmembers. The member adjacent to the Inconel flange is 0.05 inch thickand is composed of type 347 stainless steel. The member adjacent to theother flange (Cb-lZr) is 0.035 inch thick and is composed of a tantalumbase alloy (T-222). The gasket members are joined together by explosivebonding to form a bonded zone having a thickness of 0.2 mil.

When the joint is assembled in preload condition with the nut and boltassemblies and with the gasket tightened in place, the opposite ends ofthe flanges are 1.25 inches apart. The angle of inclination of thegasket joint is 47.2S.

Another embodiment of the invention is shown in FIG. 2 which differsprimarily from that of FIG. 1 in that the flanges 16 and 18 are providedwith end surfaces 110 and 112 having an arcuate or dish-shapedconfiguration. More particularly, the end surface 110 is concave and theend surface 1 12 is convex. In a preferred embodiment the end surfaces110 and 112 are concentrically disposed.

As shown in FIG. 2, the outer periphery of the surface 110 terminates ata junction 114 with the annular side surface 38. The inner extremity ofthe surface 110 terminates at a peripheral line 116 which is axiallyspaced further to the left away from the flange 18 than is theperipheral line 114.

In a similar manner the convex end surface 112 has an outer peripheralline 120 where it meets the annular side surface 52 of the flange 18.The inner periphery of the end surface 112 terminates at a line 122formed with the upper end of a surface 124.

Each flange 16 and 18 may have an enlarged body portion 126 and 128,respectively, which portions reinforce the overall joint structureincluding the end surfaces 110 and 112. As viewed in FIG. 2, the leftend of the body portion 126 terminates at an annular sidewall 130 andthe body portion 128 terminates at the right end with an annularsidewall 132.

A circular gasket 134 is disposed between the end surfaces 110 and 112where it is held secure in place by the nut and bolt assembly 22. Thegasket 134 is composed of two sheetlike layers 136 and 138 having abonded interface 140 which interface is coextensive with the endsurfaces 110 and 112. The periphery of the layers 136 and 138 preferablyterminates in alignment with junctions 114 and 120. The inner endportions of the layers 136 and 138 are of bifurcated configuration withoppositely extending arcuate portions which terminate in flanges 142 and144 which flanges are connected thereto by welds 146 and 148. In thealternative flanges 142 and 144 may be an integral part of the layers136 and 138 respectively. As shown in FIG. 2 the remote extremities ofthe flanges I42 and 144 are welded at 150 and 152 in a fluidtight mannerto the inner conduit surfaces 30 and 56 respectively.

Moreover, although the end surfaces 110 and 112 are arcuate or dishshaped, they are generally disposed in a zone that is inclined to avertical radial plane. For example, the zone between the end surfaces 1and 112 (occupied by the layers 136 and 138) extends at an angle fromthe lines 116 and 122 at the lower end of the zone to the junctions 114and 120 at the upper end of the zone. It is noted that the junctions 114and 120 are to the right of the lines 116 and 122, respectively, asviewed in FIG. 2.

The layers 136 and 138 are preferably composed of materials havingcoefficients of thermal expansion substantially equal to those of thematerials of which the adjacent members, namely the flanges 16 and 18,are composed. Thus, the flange 18, being composed of a material ofrelatively low linear thermal coefficient of expansion, has the layer138 also of low thermal coefiicient of expansion, in surface-to-surfacecontact with the end surface 112. Likewise the flange 16, having arelatively high coefficient of linear thermal expansion has the layer136 also of a high thermal coefficient of expansion, insurface-to-surface contact with the end surface 110.

In operation the joint or coupling shown in FIG. 2 functions in a mannersubstantially similar to that of FIG. 1. The arcuately disposed gasketlayers 136 and 138 also move in compensating linear and radialdirections due to thermal expansion and contractions and therebymaintain a substantially constant load on the assembly 22. Theconstruction of FIG. 2 compensates better for the case where there is athermal gradient in the radial direction and/or when there is present anonlinear coefficient of expansion with changes in temperature.

As the portion of the coupling including the flange 16 expands outwardlyat a greater rate than the portion of the coupling including the flange18, the lower bifurcated end portions of the layers 136 and 138 movefreely within the spaces 137 and 139 in closer proximity of the convexsurfaces 118 and 124. As a result the bonded interface is relieved ofmost stresses due to differences in expansion caused by the thermalgradient across the interface. Where the assembly 22 is composed of amaterial having a high expansion coefficient (comparable with that ofthe flange 16), the curved surfaces are inclined in a reverse directionto the opposite side of a radial plane; that is, on the left as viewedin FIG. 2.

Another embodiment of the invention is shown in FIG. 3 which disclosesmembers substantially similar to the embodimerits of FIGS. 1 and 2 andwhich differs primarily in the clamping means for holding the couplingtogether. The clamping means includes an annular ring 154 having aninturned flange 156 and having a threaded surface portion 158 whichengages a corresponding threaded surface 160 on the outer periphery ofthe flange 18. The annular ring 154 is composed of a material having avery low coefficient of linear thermal expansion such as that of whichthe flange 118 is composed. The clamping means including the ring 154constitutes an alternative to the nut and bolt assembly 22. An annularclearance 162 is provided between the outer extremity of the flange 16-and the ring 154. Likewise, a clearance 164 extends radially between theflanges 16 and 18 in a manner similar to the embodiments shown in FIGS.1 and 2.

Finally another embodiment of the clamping means is shown in FIG. 4which means includes an inner ring 166 and an outer ring 168. The ring116 includes an inturned flange 170 engageable with the side surface 36of the flange 16. Likewise, the ring 168 includes an inturned flange 172which engages the annular side surface of the flange 18. As shown inFIG. 4 the rings 166 and 168 have cooperating interlocking means such asthreaded surfaces 174 and 176, respectively. The rings 166 and 168 arecomposed of materials having the same or substantially similarcoefl'rcients of linear thermal expansion which material substantiallycorresponds to that of the ring 18.

Accordingly, the device of the present invention provides a combinationmetallurgical and mechanical means for maintaining a leakproof andreliable coupling between conduits of widely differing coefi'rcients oflinear thermal expansion. The device includes flanges having uniquelycontoured surfaces between which a preloaded gasket remainssubstantially unchanged throughout repeated severe thermal cycles inresponse to varying operating temperature limits. Moreover, it isunderstood that where the walls of the conduits are very thick, the endsurfaces may be inclined between the inner and outer wall surfaces andthereby obviate the need for flanges, and suitable fastening means maybe otherwise provided.

Various modifications may be made within the spirit of this invention.

What is claimed is:

1. A coupling joint between a pair of pipes of substantially differentcoefficients of thermal expansion and suitable for a coefficientdifference of about 4 microinches per inch per F., comprising first andsecond pipes having inner and outer wall surfaces, the first pipe havinga higher coefiicient of thermal expansion than the second pipe, thefirst pipe having an end surface inclined at an angle to a radial planethrough the pipe, the second pipe having an end surface inclined at anangle substantially corresponding to that of the first pipe, a gasketinterposed between the end surfaces and comprising superposed layers ofmetal having different coefficients of thermal expansion, the metallayer having the higher coefiicient of expansion being disposed adjacentto the first pipe, the layers of metal having a metallurgically bondedinterface, means for holding the inclined end surfaces insurface-to-surface contact with adjacent layers of the gasket, the meansbeing composed of a material having a coefficient substantially equal tothat of one of the first and second pipes, and the end surfaces beinginclined with respect to the pipe axes in a direction forming an acuteangle facing the pipe having a coefficient substantially equal to thatof the holding means, whereby the coupling maintains a constant jointloading between the first and second pipes in response to differentialradial and axial contractions and expansions of the pipes duringrepeated changes in temperatures of a fluid flowing through the pipes.

2. The coupling of claim 1 wherein the means for holding the inclinedend surfaces in surface-to-surface contact with adjacent layers of thegasket include clamping means having a coefficient of thermal expansionsubstantially equal to that of the second pipe, and the angles ofinclination of the end surfaces are inclined in directions extendingfrom the inner tubular wall of the first pipe to the outer tubular wallof the second pipe.

3. The coupling of claim 1 wherein the means for holding the inclinedend surfaces in surface-to-surface contact with adjacent layers of thegasket include clamping means having coefficient of thermal expansionsubstantially equal to that of the first pipe, and the angles ofinclination of the end surfaces are inclined in directions extendingfrom the inner tubular wall of the second pipe to the outer tubular wallof the first pipe.

4. The coupling of claim 1 wherein means for holding the pipe endsurfaces in contact with the gasket includes flange means extendingoutwardly from the outer wall of each pipe, and the means also includingclamping means for holding the flanges in fixed axial positions withrespect to each other during varying operating temperatures.

5. The coupling of claim 1 wherein the means for holding the pipe endsurfaces in contact with the gasket includes a peripheral flange on thefirst pipe, the peripheral flange having an annular side surface remotefrom the second pipe, the aniii nular side surface of the peripheralflange being disposed in a radial plane of the first pipe, and the angleof inclination of the gasket bonded interfaces being in a plane thatintersects the radial plane of the annular side surface substantially atthe axis of the pipe.

6. The coupling of claim 1 wherein the inclined end surfaces of thefirst and second pipes are arcuate in substantially concentricconfiguration.

7. The coupling of claim 6 wherein the arcuate end surface of the firstpipe is concave and that of the second pipe is convex.

8. The coupling of claim 7 wherein the gasket layers are arcuate inconformity with the end surfaces.

9. The coupling of claim 1 wherein the bonded interface between themetal layers has a thickness of up to about 0.5 mil.

10. The coupling of claim 1 in which the means for holding the endsurfaces and gasket together include a peripheral flange on each pipe,and tension means for holding the flanges against axial separation.

11. The coupling of claim 10, in which the tension means include nut andbolt assemblies extending through and between the flanges.

12. The coupling of claim 10 in which the tension means include clampingmeans for engaging the flanges.

13. A coupling joint between a pair of pipes of substantially differentcoefficients of thermal expansion, comprising first and second pipeshaving inner and outer wall surfaces, the first pipe having a highercoefficient of thermal expansion than the second pipe, the first pipehaving an integral first flange extending radially outwardly from theouter wall surface, the second pipe having an integral second flangeextending radially outwardly from the outer wall surface, the firstflange having an end surface inclined at an angle to a radial planethrough the pipe, the second flange having an end surface inclined at anangle substantially corresponding to that of the first flange, a gasketinterposed between the end surfaces and comprising superposed layers ofmetal, having different coefficients of thermal expansion, the metallayer having the higher coefficient of expansion being disposed adjacentto the first flange, the gasket layers having a metallurgically bondedinterface, means including bolts extending between the flanges forholding the joint together, the bolts being composed of a metal having acoefficient of thermal expansion substantially equal to that of thesecond pipe, and the angle of inclination of the end surfaces being anacute angle facing the second pipe.

2. The coupling of claim 1 wherein the means for holding the inclinedend surfaces in surface-to-surface contact with adjacent layers of thegasket include clamping means having a coefficient of thermal expansionsubstantially equal to that of the second pipe, and the angles ofinclination of the end surfaces are inclined in directions extendingfrom the inner tubular wall of the first pipe to the outer tubular wallof the second pipe.
 3. The coupling of claim 1 wherein the means forholding the inclined end surfaces in surface-to-surface contact withadjacent layers of the gasket include clamping means having coefficientof thermal expansion substantially equal To that of the first pipe, andthe angles of inclination of the end surfaces are inclined in directionsextending from the inner tubular wall of the second pipe to the outertubular wall of the first pipe.
 4. The coupling of claim 1 wherein meansfor holding the pipe end surfaces in contact with the gasket includesflange means extending outwardly from the outer wall of each pipe, andthe means also including clamping means for holding the flanges in fixedaxial positions with respect to each other during varying operatingtemperatures.
 5. The coupling of claim 1 wherein the means for holdingthe pipe end surfaces in contact with the gasket includes a peripheralflange on the first pipe, the peripheral flange having an annular sidesurface remote from the second pipe, the annular side surface of theperipheral flange being disposed in a radial plane of the first pipe,and the angle of inclination of the gasket bonded interfaces being in aplane that intersects the radial plane of the annular side surfacesubstantially at the axis of the pipe.
 6. The coupling of claim 1wherein the inclined end surfaces of the first and second pipes arearcuate in substantially concentric configuration.
 7. The coupling ofclaim 6 wherein the arcuate end surface of the first pipe is concave andthat of the second pipe is convex.
 8. The coupling of claim 7 whereinthe gasket layers are arcuate in conformity with the end surfaces. 9.The coupling of claim 1 wherein the bonded interface between the metallayers has a thickness of up to about 0.5 mil.
 10. The coupling of claim1 in which the means for holding the end surfaces and gasket togetherinclude a peripheral flange on each pipe, and tension means for holdingthe flanges against axial separation.
 11. The coupling of claim 10, inwhich the tension means include nut and bolt assemblies extendingthrough and between the flanges.
 12. The coupling of claim 10 in whichthe tension means include clamping means for engaging the flanges.
 13. Acoupling joint between a pair of pipes of substantially differentcoefficients of thermal expansion, comprising first and second pipeshaving inner and outer wall surfaces, the first pipe having a highercoefficient of thermal expansion than the second pipe, the first pipehaving an integral first flange extending radially outwardly from theouter wall surface, the second pipe having an integral second flangeextending radially outwardly from the outer wall surface, the firstflange having an end surface inclined at an angle to a radial planethrough the pipe, the second flange having an end surface inclined at anangle substantially corresponding to that of the first flange, a gasketinterposed between the end surfaces and comprising superposed layers ofmetal, having different coefficients of thermal expansion, the metallayer having the higher coefficient of expansion being disposed adjacentto the first flange, the gasket layers having a metallurgically bondedinterface, means including bolts extending between the flanges forholding the joint together, the bolts being composed of a metal having acoefficient of thermal expansion substantially equal to that of thesecond pipe, and the angle of inclination of the end surfaces being anacute angle facing the second pipe.